The present invention relates to a filtering cartridge for filtering gases, particularly hot and/or radioactive and/or corrosive gases. Such cartridges are particularly useful in installations for the incineration of nuclear waste which produce hot radioactive gases (the temperature of combustion being able to attain 1200.degree. C., the gases are generally at from 600.degree. to 1000.degree. C.). The gases are laden with solid radioactive particles (for example plutonium), carbon black and corrosive vapours (Cl.sub.2, HCl, . . . ), in a more or less large quantity depending on the nature of the waste burnt.
In order to purify these gases before rejecting them into the atmosphere, two barriers of filters are disposed in series: the first is constituted by filtering candles which must stop at least 99% of the particles of diameter of the order of 1 .mu.m, and the second is a filter of very high efficiency (V.H.E.) which stops more than 99% of the particles of diameter greater than 0.3 .mu.m. The duplicate arrangement of the barriers makes it possible to increase the life duration of the filter (V.H.E.) which is a very expensive device. The cartridges of the invention are perfectly suitable for the first of these barriers.
Such a filtering barrier with filtering candles is described in French Patent No. 1 503 631; it is schematically shown in FIGS. 1 and 2.
The filtering candles 1 are constituted by a supporting cylinder 2 and a filtering cartridge 3.
The supporting cylinder 2 is porous so as to allow the gases to pass through its walls; it is constituted for example by a wound metallic cloth of which the ends are welded. One of the ends 4 of the cylinder is closed, the other, open, end 5 comprises means for connecting the candle to a plate 6. These means are for example a sleeve 7 fixed to the cylinder and to the plate; in this case, the plate with all its candles is removed for replacement of said candles.
The filtering cartridge 3 is constituted by a felt of defibrated asbestos fibers mixed with previously calcined asbestos. These fibers ensure filtration. In order to limit shrinkage thereof when hot, they may be mixed with fibers of vitreous silica, microquartz, mullite, kaolin, . . . . The fibers of the filtering cartridge 3 are deposited on the support 2 by gaseous entrainment.
In French Patent No. 2 556 608, a candle is individually replaced, the means for connection thereof with the plate being constituted by a flange fixed on the end 5 and simply abutting on the plate.
The candles with asbestos fibers are entirely satisfactory from the standpoints of filtration and cost. However, at the present time, the administrative authorities in certain countries are limiting, and even prohibiting, the use of asbestos.
It was therefore necessary to find a substitute material:
efficient against dust (99% for dust of about 1 .mu.m) for reasonable thicknesses; PA1 with slow speed of clogging-up; PA1 resistant to thermal shocks and chemical aggressivity; PA1 of moderate price. PA1 diameter: 25 mm PA1 height: 850 mm PA1 filtering thickness: about 5 to 10 mm. PA1 either the cartridge is placed in a generally cylindrical support, the latter presenting at its open end a means for being supported by the plate. It is also provided, at the level of that end, with a means for closing the free space between the cylinder and the cartridge, so that the gases to be filtered are directed on entering the hollow space of the cartridge. PA1 or the cartridge is used as such, in that case adding thereto on its open end a means for being supported by the plate, this means obviously being designed and disposed so that the gases to be filtered are directed solely in the hollow space of the cartridge. This may be a sleeve fitted in the hollow space of the cartridge and with a flange abutting on the plate, the diameter of the recess of the plate being slightly larger than the outer diameter of the cartridge, so as to change it easily.
Furthermore, ceramic candles were voluntarily set aside, since they are too expensive and their speeds of clogging-up are still too rapid.
U.S. Pat. No. 4,500,328 describes a composite material based on refractory ceramic fibers prepared by wet process and used for the filtration of hot gases, particularly incinerator gases.
According to this Patent, said material may be obtained in two different manners.
In a first process of preparation, a very dilute aqueous solution (solid/water total &lt;1% by weight) of refractory fibers (of diameter &lt;12 .mu.m, such as fibers of zirconia , alumina, silica . . . ) and of organic binding agent (phenolic resin, starch . . . at the rate of 25 to 100% by weight of the fibers) is deposited on a perforated mould placed in vacuo. The whole is stoved, dried then demoulded.
The structure obtained is taken to a temperature of 1350.degree. C. for several hours and in an inert atmosphere, in order to pyrolyze the organic binding agent.
The composite material obtained is composed of refractory fibers connected together by carbon which comes from decomposition in an inert atmosphere of the binding agent. In the presence of oxygen, the carbon would be converted into gaseous CO.sub.2, hence the necessity of the inert atmosphere.
In a second process of preparation, a very dilute aqueous solution of the refractory fibers alone is deposited on a perforated mould placed in vacuo. After this operation, a solution of nitrate of zirconium or of yttrium is passed through. The whole is dried then is subjected to a conventional sintering.
The composite material obtained is composed of refractory fibers connected together during sintering by the fibers softening. The sintering temperatures are high: 1700.degree. C. for silica and higher than 1700.degree. C. for the other materials (zircon, alumina, . . . ).
The nitrates of zirconium or of yttrium added are stabilizers: they avoid the formation of eutectics which would lower the melting temperature of the material and would consequently be detrimental to correct sintering. This is a conventional use of stabilizers.
The filters obtained according to U.S. Pat. No. 4,500,328 are constituted by the filtering material in the form of a solid cylinder of small dimensions (diameter 19 mm, height 13 mm which also constitutes the filtering thickness) placed in a metallic support substantially of the same dimensions and provided with holes for the passage of the gases.